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Thursday, March 17, 2016

The Trouble With Butyric Acid (Nerdy)

Last night I was drinking a gose and noticed a flavor I've encountered in some beers soured by Lactobacillus: the faint flavor and aroma of vomit. Delish! It's never been overwhelming in any beer I've encountered, but does tinge the whole affair with unpleasantness. Horse blanket, a touch of compost, vinegar--all of these have their place. Vomit not so much.

I tweeted out my finding and the result was a flurry of information. The culprit here is butyric acid, actually an ester, which is indeed found in human vomit.

Butyric acid is a carboxylic acid found in rancid butter, parmesan
cheese, and vomit, and has an unpleasant odor and acrid taste, with a
sweetish aftertaste (similar to ether). Butyric acid is a fatty acid
occurring in the form of esters in animal fats and plant oils.
Interestingly, low-molecular-weight esters of butyric acid, such as
methyl butyrate, have mostly pleasant aromas or tastes. As a
consequence, they find use as food and perfume additives.

(Interestingly, it's not such a problem in beers where Brettanomyces is also present, because that wild yeast can convert the esters to more pleasant tropical-fruit aromas.)

The curious question is where it comes from. One brewer reported that it can be produced by Lactobacillus in the presence of oxygen (lacto are a anaerobic bacteria). But as I dig around in my admittedly crude fashion, it seems more likely to come from Clostridium, another anaerobic genus of bacteria. It seems like a terrifically pernicious beast; there are some species of Clostridium tolerant of boiling temperatures. Even more unsettlingly, it likes carbon dioxide:

Carbon dioxide from both fermentation and artificial introduction has been shown to have a stimulatory effect on the growth of Clostridium butyricum (as well as other bacteria such as E. coli) [11]. If sanitation issues allow for Clostridium to enter the brewery, CO2 purging may encourage butyric acid formation.

It doesn't seem like lacto is a significant factor in all of this. So why do some kettle-soured beer develop the delightful flavor of vomit? My theory is that brewers are introducing Clostridium when they use grain to inoculate their wort during kettle-souring rather than a pure culture of lacto. Grain does indeed have lacto, but it has other stuff, too. (It's why sour mashes almost always produce gross ancillary flavors like garbage and, yes, vomit.) The pH drop eventually inhibits Clostridium, but perhaps not before it's had a chance to add its own charming compound.

This is all just noodling about--the reason, I'm told, blogs were invented--and I would love love love actual scientific info should anyone be in possession of such. One blogger's half-baked theories may make for an interesting three minutes' read, but it would be preferable to actually learn what's going on.

6 comments:

Here at Baerlic, we've done about a half dozen kettle sours and have picked up some Butyric in one batch. We always use a straight lacto pitch or Nancy's yogurt—we do not use grain for fear of other critters. I believe this issue was a result of 2 possible sources:

1) For that beer, we used a blend of Nancy's yogurt and a culture from apple skins in a collab with Reverend Nats. And although we may have encountered Clostridium from the skins, the starting pH and temperature of the kettle should have inhibited that potential - http://sourbeerblog.com/fast-souring-lactobacillus/

2) We could have had aerobic bacteria fermentation due to insufficient/incomplete kettle CIP and insufficient positive CO2 pressure throughout kettle souring—both processes we have since refined. http://www.milkthefunk.com/wiki/Sour_Worting

The main way of nullifying the effects of Clostridium is to acidify the mash before souring. Clostridium produces far less Butyric Acid in conditions <pH5 than above (see Montville et al 1985: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC238436/)I personally use food grade lactic acid to decrease mash pH before souring with pure Lactobacillus. If i do get residual Butyric Acid then I generally add some Brett to the primary ferment.

I run a brewery that exclusively does sour kettle beers. I've run into 2 batches with butyric. I'd say grain is a safe source for lacto if you pre-acidify the wort to <4,5 with lactic acid and keep the wort O2-free. One batch caught butyric from the dry hops. I nicked 30 bottles from the fermenter to try bottle condition and dry hopped just after that. The batch was screwed but the first 30 bottles were great. Dirty batch of hops I suppose.

I run a brewery that exclusively does sour kettle beers. I've run into 2 batches with butyric. I'd say grain is a safe source for lacto if you pre-acidify the wort to <4,5 with lactic acid and keep the wort O2-free. One batch caught butyric from the dry hops. I nicked 30 bottles from the fermenter to try bottle condition and dry hopped just after that. The batch was screwed but the first 30 bottles were great. Dirty batch of hops I suppose.